Methods of characterizing the electrical dielectric characteristics of large, curved solid surfaces have lacked an accurate means of testing at mid-range frequencies.
Information about the complex dielectric properties at mid-range frequencies is important because the manufacturing tolerances at these frequency ranges is difficult to control and such properties can vary widely. Moreover, variations in application of such materials can change the dielectric properties. Non-destructive testing of such applications with possibly large curved surfaces requires a portable probe adjustable to a range of materials and contours.
It is known at high frequencies that electromagnetic radiation can be directed at a sample from a transmitting horn antenna to a receiving horn antenna, with the characteristics determined by the change in transmission from the calibration condition to no test specimen in between to when the test specimen impedes the transmission.
It is also known that open-ended coaxial-line probes can be inserted into liquid materials in order to test dielectric properties at mid-range frequencies, 10 MHz to 1 GHz This technique is described in T. P. Marsland, "Dielectric measurements with an open-ended coaxial probe", IEE Proceedings, Vol. 134, Pt. H, No. 4, pp. 341-348, August 1987.
Attempts have been made to make probes that will test curved solid surfaces, such as in U.S. Pat. No. 5,541,522 by Rosen et al. Such probes rely upon slender, spring-loaded pins extending out to make contact with the test surface. The accuracy of such probes is limited because the small contact area of the pins provides poor, high-impedance coupling to the test source. The coupling also changes with surface curvature.